Conventional techniques for electroplating metals onto selected portions of parts has typically been performed using step-and-repeat plating for plating selective locations or a generally faster continuous strip plating process for plating portions of parts whose intended plating areas can be arranged in an uninterrupted path. Such parts can be joined together as a lead frame that is a long continuous strip containing duplicate copies of a particular part. The lead frame can be fed through machines that perform various processes on each of the parts of the lead frame in an orderly stepwise manner.
With conventional step-and-repeat plating, a precise mask is positioned over a section of a lead frame having a series of parts. The mask can have one or more openings formed therein so that portions of the part to be plated are exposed through the one or more openings. The unmasked portions of the parts of the masked lead frame section can be exposed to a plating solution and plated with metal.
The lead frame can be negatively charged to plate the exposed areas of the lead frame when they receive plating solution such as by pouring, spraying, or brushing the plating solution from a positively charged applicator, such as a nozzle. After the unmasked portions of the parts of the masked lead frame section have been plated, a new section of the lead frame is moved to be masked and to further repeat the step-and-repeat process. Although the step-and-repeat process can be used for precision plating so that relatively little plating material is wasted, the process can be inherently slow, labor intensive, and costly.
With a conventional continuous plating system, the lead frames can be run at a constant velocity through the plating system to potentially reduce labor requirements and potentially increase throughput. The lead frame is directed into a plating tank while the parts to be plated are trapped between moving masking belts. A masking belt set defines a strip opening that exposes a selected portion of each part of the lead frame therethrough to a plating solution and a backing masking belt covers other portions of each part to prevent those portions from being plated. After plating, a re-reeler can spool the plated parts onto a reel as the parts emerge from the plating tank.
Although conventional continuous plating systems can have relatively faster throughput than the conventional step-and-repeat plating systems, they tend to be more wasteful of the plating materials. In conventional continuous plating systems, the masking belt set can shift back and forth in position orthogonal to its direction of motion, also referred to as trans-linear motion, due to tracking issues with the masking belt set and associated pulleys (e.g., pulley misalignment) and undesired lateral motion of the wheel that drives the motion of the masking belt set. This trans-linear motion can cause a shift back and forth in position of the opening in the masking belt set relative to its associated lead frame part to be masked. Consequently, if the opening was only as large as its corresponding desired portion of the part to be plated, this desired portion of the part may not be fully plated. Through the trans-linear shifting, the opening may not be properly positioned over the part at the time of plating. Rather, the opening may be slightly out of position and if the opening was only the size of the desired portion of the part to be plated, not all of the desired portion of the part would be exposed through the opening to receive the plating solution.
To compensate for this shifting due to the trans-linear motion, the opening may be enlarged enough so that no matter where an opening is in its back and forth trans-linear motion, the entire desired portion of the part to be plated is still exposed through the opening to receive the plating solution. However, since the opening in the masking belt set is larger than the desired portion of the part to be plated, areas of the part that do not require plating will be over-plated, which wastes the expensive plating metal such as gold.
Some conventional continuous plating systems employ masking belts of relatively greater thickness to possibly reduce the amount of trans-linear motion. However, increased masking belt thickness can inhibit the thickness of the plating near the edge of the opening and is often referred to as the “wall effect.” The plating material on a plated portion of a part exhibits a non-uniform thickness, with the thickness being thinner near the edges of the plated portion and being thicker near the center of the plated portion. Resultant uneven plating can also waste plating material because more plating material may need to be used in a center of a plated portion in order to have a sufficient amount of plating material near the edges of the plated portion.